Diaphragm valves are known which are constructed from plastic materials, see for example CH-PS 532738, which comprise housing parts which are also constructed from plastic materials and having an elastomeric diaphragm lying thereinbetween. The housing parts are fastened together by means of metallic bolts.
In pipeline systems of high purity design which are, for example, utilized in the chemical industry in applications where the temperature of the medium is approximately 20.degree. C., it is usual to sterilize the pipeline systems periodically with superheated steam at a temperature above 120.degree. C. All of the components and parts utilized in the system and which are in contact with the medium are heated to this high temperature.
Due to the coefficients of thermal expansion of the metal and the plastics parts, which typically differ by a factor greater than 10, the extension or elongation of the components or parts of the system must be absorbed by the most pliable component of the system, namely, an elastomeric diaphragm which is additionally compressed due to the heating.
Upon termination of the sterilization process, the system once again operates at its normal temperature. The elastomeric diaphragm of the prior art systems, however, usually retains a residual deformation due to its material composition. In this manner, the diaphragm which has been pressed together at its attachment points will not expand to its original form and thickness after cooling.
In this manner, the valve, which is typically sealed tightly up to a pressure of 10 bar, can spring leaks towards the exterior of the system at a lower medium pressure. This tendency to spring leaks requires the re-tightening of the attachment bolts utilized in the system. Further, through the increasingly more pronounced deformation of the diaphragm, its elastomeric texture is damaged and therefore, its useful life is correspondingly reduced.